1. Population Growth
SC.912.L.17.5

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Population Growth
What factors affect population size?
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Population Growth
Three factors can affect population size:
the number of births
the number of deaths
the number of individuals that enter or leave the population
A population can grow when its birthrate is greater than its death rate.
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Population Growth
Immigration, the movement of individuals into an area, is another factor that can cause a population to grow. Emigration, the movement of individuals out of an area, can cause a population to decrease in size.
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Exponential Growth
What are exponential growth and logistic growth?
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Exponential Growth
Under ideal conditions with unlimited resources, a population will grow exponentially.
Exponential growth occurs when the individuals in a population reproduce at a constant rate.
The population becomes larger and larger until it approaches an infinitely large size.
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Exponential Growth
Exponential Growth
In the presence of unlimited resources and in the absence of predation and disease, a population will grow exponentially. Both hypothetical graphs show the characteristic J-shape of exponential population growth.
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Logistic Growth
As resources become less available, the growth of a population slows or stops.
Logistic growth occurs when a population's growth slows or stops following a period of exponential growth.
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Carrying Capacity: the maximum number of individuals the environment can hold
This graph shows the S-shaped curve of logistic growth. As resources become less available, the population growth rate slows or stops. The growth of this population has leveled off at its carrying capacity.
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Limiting Factors
What factors limit population growth?
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Limiting Factors
The primary productivity of an ecosystem can be reduced when there is an insufficient supply of a particular nutrient.
Ecologists call such substances limiting nutrients.
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Limiting Factors
A limiting nutrient is an example of a more general ecological concept: a limiting factor.
In the context of populations, a limiting factor is a factor that causes population growth to decrease.
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13. Density-Dependent Factors
A limiting factor that depends on population size is called a density-dependent limiting factor.
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14. Density-Dependent Factors
Density-dependent limiting factors include:
competition
predation
parasitism
disease
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15. Density-Dependent Factors
Density-dependent factors operate only when the population density reaches a certain level. These factors operate most strongly when a population is large and dense.
They do not affect small, scattered populations as greatly.
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16. Density-Dependent Factors
Competition
When populations become crowded, organisms compete for food, water space, sunlight and other essentials.
Competition among members of the same species is a density-dependent limiting factor.
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17. Density-Dependent Factors
Competition can also occur between members of different species.
This type of competition can lead to evolutionary change.
Over time, the species may evolve to occupy different niches.
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18. Density-Dependent Factors
Predation
Populations in nature are often controlled by predation.
The regulation of a population by predation takes place within a predator-prey relationship, one of the best-known mechanisms of population control.
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19. Density-Dependent Factors
Wolf and Moose Populations on Isle Royale
The relationship between moose and wolves on Isle Royale illustrates how predation can affect population growth. In this example, the moose population was also affected by changes in food supply, and the wolf population was also affected by disease.
Moose Wolves
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20. Density-Dependent Factors
Parasitism and Disease
Parasites can limit the growth of a population.
A parasite lives in or on another organism (the host) and consequently harms it.
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21. Density-Independent Factors
Density-independent limiting factors affect all populations in similar ways, regardless of the population size.
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22. Density-Independent Factors
Examples of density-independent limiting factors include:
unusual weather
natural disasters
seasonal cycles
certain human activities—such as damming rivers and clear-cutting forests
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